The application of rf radiation to a sample and the pick up of resulting resonant signal is accomplished in a structure surrounding the sample which may be a helical coil, saddle coil, resonant cavity, or a bird cage resonator. The latter structure is the object of the present work, wherein it is desired to obtain multiply resonant frequencies in the same bird cage type structure to facilitate studies of diverse chemical constituents which may be appropriate to the same sample. A bird cage coil is a ladder circuit which closes on itself wherein the current flow around the coil is distributed sinusoidally. As a tuned rf circuit, it is employed in nuclear magnetic resonance apparatus for either or both of the functions of rf excitation and signal detection. Although such coils are well known, all examples known are singly tuned.
The bird cage coil differs in essential matter from saddle coils, helices and like structures by its discrete structure. For the bird cage coil, there is a requirement that the phase shift be discretely distributed around the circumference of the coil from zero to 2.pi. (or 2.pi.n, where n is an integer). The phase shift of each element is quite frequency dependent and as a consequence, the bird cage coil is tuned at a discrete frequency to achieve the desired phase shift constraint. It is desirable to achieve a multiple tuned bird cage coil in order to obtain data at more than one resonant frequency, either concurrently or in separate measurements.
The bird cage coil is particlarly well suited to large volume samples as are routinely encountered with apparatus for medical imaging and in vivo analytic spectroscopy. Bird cage coils are discussed by Hayes et al, J. Mag. Res., vol. 63, pp. 622-628 (1985).
The bird cage structure may be regarded as a periodic structure which closes on itself. Periodic elements of the structure produce phase shifts which must aggregate to some multiplie of 2.pi. when summed over the closed loop. Geometrically, the resonator has cylindrical symmetry and it is desired that the rf current in the axial direction along the periphery of the structure be proportional to sin .theta. where .theta. is the azimuthal angle about the cylindrical axis.